Power transistors are widely used for switching electrical currents in automotive or industrial applications. For example, power transistors can be employed in any kind of switched-mode power supply, in drive circuits for driving loads, such as lamps, inductive valves, or motors, in inverters, or the like. In order to regulate the current through the load or in order to simply detect an overload scenario it is desired in many applications to measure the current flowing through the transistors.
Different concepts are known for measuring a load current through a transistor. These concepts include, e.g., connecting a shunt resistor in series with the load path of the transistor and measuring the voltage across the resistor, measuring the current using a Hall sensor, or inductively measuring the current using an inductive current sensor.
Power transistors usually include a plurality of identical transistor cells connected in parallel. According to a further concept, at least one of these transistor cells (sense cell) is used to measure the current through the other transistors cells (load cells) of the transistor. The sense cell is operated in the same operation point as the load cells (but only the current through the sense cells is measured), so that the current through the sense cell is proportional to the overall load current through the load cells. The proportionality factor is given by the ratio between the number of sense cells and the number of load cells.
There is a further need to accurately measure the current through a power transistor with low measuring losses.